1. Field of the Invention
The present invention relates to an optical disk for recording information by energy beam irradiation and particularly to an optical disk recording method having an excellent effect on various high-density recordable optical disks different in composition and recording mechanism, and an optical disk device using the recording method.
2. Description of the Related Art
A DVD-RAM using a phase change material for achieving a recording capacity of 4.7 GB per surface of a 120 mm-diameter disk has been put into practice in recent years. In the DVD-RAM, a recording film is provided with a first state (mark) and a second state (space). A predetermined repetitive pattern of the first state and the second state is formed to perform mark edge recording of information. A way of changing the level of irradiation power with the passage of time in the case where an energy beam is applied on the recording film to record information in the recording film is generally called recording strategy.
The recording strategy of the DVD-RAM is as shown in FIG. 2. The energy beam is pulsated so that the level of the energy beam changes among a first power level (recording power level Pw) for obtaining the first state, a second power level (bias power level Pb2) for obtaining the second state and a third power level (bias power level Pb1) lower than the first and second power levels. Particularly when a recording mark as the first state needs to be formed, the recording film is irradiated with a multi-pulse chain having light pulses of the first power level and light pulses of the third power level arranged alternately in accordance with the length of the recording mark in order to prevent the recording mark from being distorted geometrically.
Incidentally, if the distance between positions irradiated with two recording pulses adjacent to each other is relatively small compared with the light spot size of the energy beam applied on the recording film, there is a high possibility that both geometrical mark distortion and mark edge shift will occur because the light intensity distributions of the two recording pulses overlap each other. If the mark or space is too short, there is a high possibility that mark edge shift will occur in the waveform of a playback signal because the mark or space cannot be discriminated sufficiently on the basis of a playback light spot.
To solve the problem of mark edge shift, a correction technique has been disclosed and put into practice (e.g. see U.S. Pat. No. 5,490,126). In the correction technique, the aforementioned recording strategy based on a multi-pulse chain is used so that a light pulse of the first power level having a predetermined pulse width is applied particularly on each of leading and trailing portions of each mark, and that recording is performed while the positions of the leading and trailing portions of the mark are changed at any time in accordance with the respective lengths of the mark and preamble and following spaces to be recorded.
The way of generating the mark edge shift largely depends on the design of the recording film. A recording strategy adapted to a specific recording film is not always adapted to another recording film. Therefore, a correction technique using the recording strategy based on a multi-pulse chain has been disclosed and put into practice to cope with various recording films (U.S. Pat. No. 6,160,784). That is, one case is selected, in accordance with the disk, from a first case where the pulse rise timing of the first pulse applied on the leading portion of each mark is changed at any time in accordance with the recording mark length and preceding space length while the pulse fall timing of the first pulse is fixed and a second case where the pulse rise timing of the first pulse and the pulse fall timing of the first pulse are changed at any time in accordance with the recording mark length and preceding space length while the width of the first pulse is fixed. The last pulse applied on the trailing portion of each mark is generated in the same manner as the first pulse. That is, one case is selected, in accordance with the disk, from a first case where the pulse fall timing of the last pulse is changed at any time in accordance with the recording mark length and following space length while the pulse rise timing of the last pulse is fixed and a second case where the pulse rise timing of the last pulse and the pulse fall timing of the last pulse are changed at any time in accordance with the recording mark length and following space length while the width of the last pulse is fixed.
The density of an optical disk has kept on increasing recently with the advance of increase in quantity of data to be used. A DVD having a capacity of about 4.7 GB (gigabytes), inclusive of the aforementioned DVD-RAM, has generally come into wide use as against a CD having a capacity of about 700 MB (megabytes). A next-generation optical disk having a large capacity of 20 GB or more capable of recording high-definition images for 2 hours has been further developed and commercialized. In the next-generation optical disk, a semiconductor laser with a short wavelength band of 405 nm (blue violet) is used as a light source and the numerical aperture of an objective lens is improved to 0.85. Moreover, the modulation code is changed from EFM-plus used in the DVD to 1-7 PP modulation. In expression in run length limited code, the modulation code used in the next-generation disk is RLL(1, 7) whereas the modulation code used in the DVD is RLL(2, 10). According to the change of the modulation code, the range of change of the mark/space length used in the next-generation disk is from 2Tw to 8Tw whereas the range of change of the mark/space length used in the DVD is from 3Tw to 11Tw when Tw is the width of a data detection window.
FIG. 5 shows various characteristics in each code in the case where the transfer time T is 24 ns per user bit. Since the smallest mark/space length has a tendency toward decreasing though the detection window width has a tendency toward increasing, there is a problem that the next-generation optical disk is different from the DVD-RAM in power levels and recording strategy necessary for recording an optimum mark. Moreover, since the smallest mark/space length is reduced relative to a playback light spot, there is a problem that mark edge shift becomes more remarkable because of reduction in resolving power.
The smallest mark length in a linear direction in the next-generation optical disk is 0.08 μm whereas the smallest mark length in a linear direction in the DVD-RAM is 0.28 μm. The next-generation optical disk has a structure in which adjacent marks/spaces are closer to each other physically. For this reason, there is a problem that thermal interference is easily caused by energy injected at the time of recording a mark. Particularly when a high-speed recordable recording medium will appear in the future, it is preferable that the recording medium can be used in a low-speed recording apparatus in terms of downward compatibility. It may be foreseen that the problem of thermal interference will become more serious because the high-speed recordable recording medium must have heat storage characteristic as a result of improvement in recording sensitivity. For this reason, there is a problem that mark edge shift at the time of recording becomes more remarkable.